High speed computation is the critical design factor in many systems, such as computers, signal processors, and process controllers. These systems increasingly rely on LSI integrated circuits to perform the multiplicative function on a single chip. Multiplication using signed digit redundant number representation allows the addition of partial products in the binary tree at speeds faster than the Booth-Wallace approach, and at the same time, has an iterative structure that increases circuit density and ease of layout. The signed digit redundant number representation permits high speed binary addition trees, because it is possible to perform carry-free addition therewith.
In order to take advantage of the binary tree, a circuit cell is needed that will sum two signed digits. Such circuits have been described for ECL technology in N. Takagi, H. Yashuura, S. Yajima, High Speed VLSI Multiplication Algorithm with a Redundant Binary Addition Tree, IEEE Transactions on Computers, Vol. C-34, No. 9, Sept., 1985. The structure described therein is designed for ECL circuitry using NOR/OR logic, which is inapplicable to other technologies, such as CMOS. Further, the circuit described therein does not minimize the number of gate delays and transistors and maximize speed in performing the logical addition of two signed digits.
Therefore, a need has arisen in the industry for a signed digit adder circuit which minimizes gate delays and the number of transistors used to implement a signed digit adder circuit.